I have a Moto G5 Plus running Oreo 8.1. Currently, the phone is connected to an Asus RT-ACRH13 router with 1 Gig service. I know that the phone is only capable of running off of N and not AC, but I’ve only been able to see speeds of 150 Mbps. I have also noticed the phone seems to be capped at 150 Mbps no matter which network/router I’ve connected to.

The Wireless chipset hardware of the device it self determines the maximum link speed possible. For that device with an N chip, it is 150Mbps. For other phones, like the Pixel that has an AC chip, it is 866Mbps.